1. Field of the Invention
The invention relates to a variable capacitor including control electrodes to change capacitance thereof. More specifically, the invention relates to a variable capacitor in which control electric field independently controls signal electric field, and an electronic device utilizing such a variable capacitor.
2. Description of the Related Art
Variable capacitors are widely utilized in electronics for controlling frequency and time by externally applying bias signals thereto in order to vary the capacitance thereof, and are commercially produced, for example, as diodes (variable capacitance diodes) or MEMS (Micro Electro Mechanical Systems). The variable capacitors of this kind include no dedicated terminals for applying bias signals to control variable capacitance. That is, the variable capacitor includes commonly used two types of terminals only, namely, signal terminals and control terminals (two-terminal device). When the variable capacitor is used in a real circuit, the variable capacitor needs to have to four types of terminals as a circuit configuration shown in FIG. 1.
FIG. 1 shows an example of an equivalent circuit including a related art variable capacitor 150 with four types of terminals. In this example of the related art equivalent circuit, the variable capacitor 150 and a bias eliminating capacitor 151 are connected in series. In the equivalent circuit, a first electrode of the variable capacitor 150 is connected to a first electrode of a bias eliminating capacitor 151. In the equivalent circuit, the control voltage is applied via a resistor R to an interconnect connected between the first electrode of the bias eliminating capacitor 151 and the first electrode of the variable capacitor 150.
In the equivalent circuit, an AC signal flows in the bias eliminating capacitor 151 and the variable capacitor 150. A DC bias current flows only in the variable capacitor 150 via resistance R. As shown by the example of FIG. 1, a signal voltage source transmitting the AC signal and a control voltage source for the DC bias current are separately provided in the equivalent circuit.
However, although the control voltage source and the signal voltage source are independently provided in the equivalent circuit, the terminals are not independently provided for the signal voltage and the control voltage in the variable capacitor 150.
In the equivalent circuit having such a configuration, the AC signal is interfered with the DC bias current flown from the control voltage source.
Japanese Unexamined Patent Application Publication No. 2007-287996 has, for example, disclosed a variable capacitor including a ferroelectric material. The publication discloses the variable capacitor including an electrode structure with improved reliability and productivity. The disclosed variable capacitor is described with reference to FIGS. 2A, 2B. FIG. 2A is a schematic perspective diagram of the variable capacitor, and FIG. 2B is a schematic configuration diagram thereof.
The variable capacitor 100 disclosed in the publication includes a cubic dielectric layer 104 with four surfaces, each of which includes a terminal formed thereon as shown in FIGS. 2A, 2B. Two of the four terminals are signal terminals 103 connected to the internal signal electrodes whereas the remaining two are control terminals 102 connected to the internal control electrodes. The internal structure of the variable capacitor 100 is configured such that pluralities of signal electrodes and control electrodes are alternately provided via the dielectric layer 104 in a layered manner as shown in FIG. 2B. In the example of FIG. 2B, control electrodes 102a indicate the bottom-layer electrode, the fifth-layer electrode from the bottom, and the top layer-electrode each connected to one of the control terminals 102 whereas control electrodes 102b indicate the third-layer electrode from the bottom, and the seventh-layer electrode from the bottom each connected to the other control terminal 102. Further, signal electrodes 103a indicate the second-layer electrode from the bottom, and the sixth-layer electrode from the bottom each connected to one of the signal terminals 103 whereas signal electrodes 103b indicate the fourth-layer electrode from the bottom, and the eighth-layer electrode from the bottom each connected to the other signal terminal 103.
In the related art example of the variable capacitor, the capacitance thereof can be increased at low cost by providing the plurality of signal electrodes 103a, 103b, and the plurality of control electrodes 102a, 102b via the dielectric layer 104 in a layered manner, while the voltage can be independently applied to the control terminals 102 and signal terminals 103.
As described above, it is easy to fabricate the variable capacitor 100 at low cost by providing the plurality of control electrodes 102a, 102b and the plurality of signal electrodes 103a, 103b via the dielectric layer 104 in a layered manner. However, the signal capacitance between the signal terminals 103, and the control capacitance between the control terminals 102 are, as shown by C1 to C8 in FIG. 2B, created in the same direction in the dielectric layer 104. In this case, the signal electric field and control electric field is generated in the same directions.
Thus, in the related art variable capacitor 100, when thin dielectric layers are provided to increase the sensitivity of the electric field so as to change the capacitance value by the application of low control voltage, an insulating property decreases and lower pressure resistance to the signal voltage.
Specifically, the signal electric field is generated in parallel with the control electric field so that the signal capacitance created between the signal terminals 103 and the control capacitance created between the control terminals include the same capacitance. In this case, there is a reciprocal relationship between lowering the control voltage and increasing the pressure resistance to the signal voltage in the variable capacitor. Accordingly, it may be difficult to control the capacitance of the related art variable capacitor 100 by applying low control voltage while applying signal voltage with large amplitude.